My daughter was admitted to hospital a few weeks ago and diagnosed with gastritis. the symptoms passed after a few days but now the pain is back, what should we do.

There are many: MANY causes of abdominal pain, gastritis is one of the more common causes. Have her go see a GI specialist. In the meantime, try PPI such as omeprazole. It should help if pain is caused by acid over production causing gastritis. If she is unable to eat/drink, develops diarrhea, fever, chills, or worsening pain especially in lower abdomen, she needs to go to ED, it could be appendicitis.